1. Field of the Invention
The present invention relates to an organic electroluminescent (EL) device employing a multi-layered anode, and more particularly, to an organic EL device having improved emission efficiency by employing a multi-layered anode having good reflectivity and high hole injecting efficiency.
2. Description of the Related Art
An organic EL device is generally constructed such that a hole injection electrode is arranged on a substrate, e.g., a glass substrate, organic layers for emitting light are stacked on the hole injection electrode, and an electron injection electrode is stacked on the organic layers. An active matrix (AM) drive type organic EL device is constructed such that a thin film transistor is arranged on a substrate, and a hole injection electrode, organic layers and an electron injection electrode are sequentially stacked thereon.
When the stacked organic EL device is a rear emitting type, light is emitted from a transparent substrate. In an EL device having a built-in pixel driver circuit, since a wide area of a substrate is occupied by the pixel driver circuit, a light emitting area, i.e., an aperture ratio, is sharply reduced. Such a reduction in aperture ratio increases current density, which may adversely affect performance of an organic EL device.
To solve the above-described problems, a front emitting type organic EL device has been proposed. In the case of the front emitting type organic EL device, a wide area may be used as a light emitting area irrespective of an area occupied by a thin film transistor circuit.
An example of a conventional organic EL device of a reflection type is disclosed in U.S. Pat. No. 5,796,509, in which a light emitting film which emits light by an electronic signal is disposed between two electrodes, that is, a transparent electrode and a patterned reflective electrode, the light emitted from the light emitting film passing through the transparent electrode to display an image, and the light emitted toward the reflective electrode being reflected to the patterned reflective electrode to display an image.
U.S. Pat. No. 6,016,033 discloses an organic EL device having a substrate, a first conductive layer formed on the substrate, an organic emission layer formed on the first conductive layer, a second conductive layer formed on the organic emission layer, and a barrier layer between the first and second conductive layers. In the disclosed organic EL device, the first conductive layer serves as both an electron injection layer and a reflection layer. The second conductive layer serves as a hole injection layer.
U.S. Pat. No. 5,714,838 discloses an organic EL device having a cathode and an anode sequentially formed on a substrate, an organic layer disposed between the cathode and the anode and emitting light according to voltage application, and a barrier layer interposed between the organic layer and an adjacent electrode. In the organic EL device, the cathode has a low work function and a good electron injection capability, and the anode and the barrier layer have a high work function and a good hole injection capability. In an embodiment of the disclosed patent, an anode made of, for example, ITO or Al is formed on an opaque substrate, e.g., a silicon substrate, and a cathode is formed using a transparent material such as ZnS, GaN, ITO or ZnSe.
The use of Al for forming an anode greatly increases a driving voltage, making it difficult to be used in an organic EL device in practice.
A top-emitting structure has been proposed to maximize an aperture ratio and improve emission efficiency (SID Digest pp 384–386, Title: “A 13.0-inch AMOLED display with top emitting structure adaptive current mode programmed pixel circuit”). According to the technique, a front-emitting organic EL device includes a stacked structure in which a reflective anode, a hole transport layer, an emission layer, an electron transport layer and a semi-transparent cathode are sequentially stacked, and the light from the emission layer is emitted through the semi-transparent cathode to drive the device. Pure R, G and B color emissions produced from such a structure are subjected to multiple interference due to partial reflectance occurring at the semi-transparent anode. Optimization of such a multi-layered structure improves color coordinate characteristics.
As the reflective anode, a single metal layer is used. The use of the single metal layer as an anode reduces the reflectivity of the anode to less than 60%, deteriorating a utilization efficiency of emitted light, resulting in a rise in driving voltage, and lowering a lifetime of the device.
As described above, the conventional methods of fabricating a front-emitting organic EL device, have several problems. That is, since the reflectivity of an anode is not taken into consideration, or a single metal layer used as a reflective layer has low reflectivity, about half of the light emitted from an organic emission layer cannot be efficiently utilized and is lost. In particular, use of Al in fabricating an anode disadvantageously lowers the driving voltage of the device, making it difficult to actually employ the Al anode in an organic EL device.